DESCRIPTION: (Applicant's Abstract) The long-term goal of this research is to develop a successful approach to in utero stem cell transplantation in human fetuses diagnosed with immunodeficiency diseases, hemoglobinopathies, or inborn errors of metabolism. In an in utero murine model of hematopoietically non-defective mice, while 70% engraft, the percent of donor cells is low (.001-2%). However, most engrafted animals are partially tolerant to donor skin grafts; some show permanent skin graft acceptance, and a few have no evidence of tolerance to skin grafts. The specific aim of this application is to define the mechanisms that are responsible for the in utero induction of tolerance to allogeneic donor cells. The hypotheses that will be tested are: 1) The limited number of allogeneic mismatched hematopoietic stem cells (HSC) that engraft in utero can be expanded and differentiate into multilineages in non-defective recipients, and 2) tolerance of the host to allogeneic mismatched HSC is mediated by marrow derived donor dendritic cells (DC) and T cells. Cytokine-recruited peripheral blood stem cells will be used to determine the optimal number of cells required to achieve engraftment, and engrafted cells in the thymus, spleen, lymph node, blood, liver, and marrow will be phenotyped. Experiments will be carried out to determine whether deletion in the thymus (or periphery) is involved in tolerance induction in engrafted mice and what role anergy plays centrally and/or in the periphery. Results of the proposed experiments will lead to the development of therapeutic strategies for the optimization of engraftment of donor cells pre and postnatally and could have an impact on other areas of transplantation, specifically, induction and maintenance of tolerance to solid organs.